The invention pertains to control systems and, more particularly, by way of non-limiting example, to fault-detecting and fault-tolerant methods and apparatus for process control.
The terms “control” and “control systems” refer to the control of the operational parameters of a device or system by monitoring one or more of its characteristics. This is used to insure that output, processing, quality and/or efficiency remain within desired parameters over the course of time.
Control is used number of fields. Process control, for example, is typically employed in the manufacturing sector for process, repetitive and discrete manufactures, though, it also has wide application in electric and other service industries. Environmental control finds application in residential, commercial, institutional and industrial settings, where temperature and other environmental factors must be properly maintained. Control is also used in articles of manufacture, from toasters to aircraft, to monitor and control device operation.
Control systems typically utilize field devices that are physically integrated into the equipment being controlled. For example, temperature sensors are usually installed directly on or within the articles, bins, or conduits that process, contain or transport the materials being measured. Control devices such as valves, relays, and the like, must also be integrated with the equipment whose operations they govern.
Predictability is among the key requirements of any control device. A fluid sensor that even occasionally produces flaky readings is unacceptable. Overengineering can insure better reliability; however, it often results in devices that are too expensive or too large for wide application.
Redundancy is a well accepted alternative to overengineering. It typically involves using two or more standard control elements in place of one. The duplicated units can be field modules, controllers or other higher-level elements in the control hierarchy.
Thus, for example, U.S. Pat. No. 4,347,563 discloses an industrial control system in which redundant processing units serve as bus masters “of the moment,” monitoring status information generated by primary processing units. If a redundant unit detects that a primary has gone faulty while executing an applications program, the redundant unit loads that program and takes over the primary's function.
U.S. Pat. No. 5,008,805, on the other hand, discloses a real time control system in which “sender” and “listener” processors synchronously step through sequential schedules, with the sender controlling execution of events sent from a host. The listener monitors the sender and in the event of fault, assumes the role of the latter, executing commands omitted during the takeover interval.
A shortcoming of these and many other prior art redundancy schemes is their imposition of undue computational or hardware overhead. U.S. Pat. No. 5,008,805, for example, has the disadvantage of requiring that the sender and listener operate in lock-step, necessitating common timing lines and up-front synchronization procedures.
An object of this invention is to provide improved methods and apparatus for control and more particularly, for example, for fault-tolerance and fault-detection in process control.
A related object of the invention is to provide such methods and apparatus as demand low computation and hardware overhead and thus, for example, that can be implemented on a wide range of control equipment, from field devices to plant and enterprise servers.
A further object of the invention is to provide such methods and apparatus as can be implemented in new control equipment and retrofitted into pre-existing equipment.